In the present day design of power amplifiers for driving loudspeakers and the like, the common practice is to utilize an operational amplifier in the front end of the audio system. The advantages of utilizing such an operational amplifier include the permissible use of high levels of feedback, the production of very low output off-set voltages, and a relatively high input impedance. This impedance remains constant with overload and frequency variation. In addition, the use of an operational amplifier provides an economic advantage since it is available in a single integrated circuit. However, as a typical integrated circuit has relatively low supply limits, a transistor gain stage must be utilized following the operational amplifier to bring the signal level up for driving the traditional emitter follower type circuits of the complementary or quasi-complementary designs. In such circuit designs, it is quite common to find supply voltages at eighty volts or higher. Now the operational amplifier will remain in the linear mode as long as the output of the power amplifier follows the input signal in a nondistorted manner.
In high-powered audio design, it is extremely advantageous to prevent the power amplifier from "clipping" which causes an extremely harsh sounding square wave to be delivered to the speaker system. These square waves are clipped sine waves and the flat top portion of these clipped wave forms represent D.C. delivered to the speaker system. These square waves, producing excessive RMS power levels, substantially increase the probability of damage to the loudspeaker because of the increased heat dissipation as well as requiring the diaphragms of the speakers to change directions almost instantaneously because of the steep wave form instead of the more gradual movement allowed for by the sine wave type reproduction.
If output clipping takes place because signal conditions exceed supply voltages or V/I limiting circuits become operational due to overloads or short circuits or slew rate limiting occurs on high slewing type signals, the input amplifier will immediately generate a nonlinear signal which will cause the output of the operational amplifier to swing to either supply rail and stay there until signal conditions are again linear. Once linear operation is established, the operational amplifier must recover at its slew rate speed back to near "0" to regain control of the amplifier. During this time, there is a so-called "dead zone" which is a nonlinear type situation presenting a common problem generally referred to as transient intermodulation (T.I.M.) distortion. To minimize such signal distortion, it is necessary to prevent the operational amplifier from going to the supply rails which is a relatively difficult action to prevent. Some progress toward solving this distortion problem has been obtained by adjusting the gain stage to have the same gain stage as the feedback network feeding the operational amplifier so that the operational amplifier will then operate at unity gain, i.e., the input port, feedback port, and output port are all at the same signal level as long as linear operation is maintained. Then, after decoupling to achieve "0" D.C. conditions, a pair of silicon diodes connected in oppositely poled relationship is connected between the operational amplifier output port to the feedback port. Thus, with this arrangement, when the operational amplifier will take less time to recover so that significantly less T.I.M. distortion is encountered. However, while such a system provides a partial solution to signal distortion, there still remains the problem of amplifier clipping caused by overdrive conditions which produce undesirable audible distortion for which no solution has been provided in present-day amplifier systems.